1. Field of the Invention
The present invention relates to a thin film transistor (TFT). More particularly, the present invention relates to a TFT, in which source and drain electrodes having concentric circular shapes are formed. The TFT may reduce a leakage current or an OFF current caused by a parasitic TFT capacitance and optimize an ON current and a stray capacitance between gate and source electrodes.
2. Discussion of the Related Art
Liquid crystal display devices include flat display devices. Flat display devices are lightweight and have a thin profile and a low power consumption rate. Because of these characteristics, the flat display device is used in various fields.
In a liquid crystal display device, switching elements are formed at pixels, and the pixels are respectively operated by the switching elements. TFTs are used as the switching elements.
FIG. 1A is a top view of a related art TFT used in a liquid crystal display device, and FIG. 1B is a sectional view taken along the line I-I′ of FIG. 1A.
As shown in FIG. 1A, the related art TFT comprises ends of an amorphous silicon layer that protrude from source and drain electrodes and are formed on a gate electrode.
The ends of the amorphous silicon layer may cause several problems, such as difficulty in finishing fine shapes and controlling cleanliness. Also, a leakage current that is due to of a parasitic TFT with contaminants (for example, remaining resist and residue formed during a dry etching process) may be generated.
When the above leakage current is high, the ability to hold charges is deteriorated. Also, flickering of an image or an after-image occurs.
Further, when a stain due to leakage current occurs on a screen, a stain in the image due to gradation of light occurs.
In order to solve the above problems caused by the leakage current, various solutions for reducing leakage current in a TFT have been suggested.
For example, Japanese Patent Laid-open Publication No. 2004-48036 discloses TFTs having a low leakage current. FIGS. 2A and 2B respectively illustrate the related art TFTs disclosed by the Patent.
As shown in FIGS. 2A and 2B, electrodes having concentric circular shapes are disposed on active layers 504 and 508. Gate electrodes 502 and 506 respectively surround electrodes 501 and 505. Electrodes 503 and 507 respectively surround the gate electrodes 502 and 506. The electrodes 501 and 505 are disposed in different layers from the layers in which wiring metals of the gate electrodes 502 and 506 are disposed, and the electrodes 501 and 505 and the electrodes 503 and 507 are disposed in the same layers.
One of the electrodes 501 and 503 and one of the electrodes 505 and 507 serve as source electrodes, and the other one of the electrodes 501 and 503 and the other one of the electrodes 505 and 507 serve as drain electrodes.
According to the above structures, the related art TFTs do not include an amorphous silicon layer with protruding ends. Thus, a leakage current may be reduced.
Further, Korean Patent Laid-open Publication No. 10-2005-0006340 discloses a related art TFT having electrodes having concentric circular shapes.
FIG. 3A is a schematic view of the TFT, disclosed by the above Patent, which is disposed in a display panel, and FIG. 3B is a sectional view taken along the line VIIIb-VIIIb′ of FIG. 3A.
Gate electrodes 124 and gate lines 121 are disposed on a substrate 110, and an intrinsic amorphous silicon layer 154 is formed thereon by interposing a gate insulating layer between the gate electrodes 124 and the intrinsic amorphous silicon layer 154. Then, circular electrodes 175 and electrodes 173 surrounding the circular electrodes 175 are disposed thereon such that the electrodes 175 and 173 are disposed in the same layer.
One of the electrodes 175 and 173 serves as a source electrode, and the other one of the electrodes 175 and 176 serves as a drain electrode.
The electrodes 175 and 173 have concentric circular shapes, thereby structurally removing protruded ends of the intrinsic amorphous silicon layer 154.
As described above, the related art TFTs do not include an amorphous silicon layer with protruded ends. Thus, leakage current may be reduced.
Further, the related art TFT causes a level shift of the electric potential of a different display electrode. The level shift is caused by a stray capacitance generated by overlapping between an electrode and a gate electrode of the related art TFT.
The amount of the level shift is in proportion to the stray capacitance. In order to prevent flickering of an image or occurrence of an after-image, the stray capacitance should be small and the distribution of the electrodes 183 should be narrow.
The above related art TFTs reduce leakage current, but cannot solve the level shift of the electric potential of the display electrode.
Accordingly, a technique for simultaneously reducing a leakage current and removing a level shift is required.
Further, in order to allow large-sized liquid crystal TVs, increase of an ON current is necessary. That is, a method for optimizing three factors, i.e., reduction of a leakage current, increase of an ON current, and decrease of a stray capacitance, is required.